Throughout this application, various publications are referenced by author and date. Full citations for these publications may be found listed alphabetically at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed herein.
The use of murine model to investigate human cutaneous oncology, immunology and keratinocyte biology is advantageous over the use of human skin for obvious reasons. However, substantial differences exist between human skin and murine skin. In human skin, Stem Cell Factor is produced by epidermal keratinocytes after birth, unlike in normal murine skin. The result of this, among other things, is that melanocytes are present in the interadnexal epidermis in human skin. In contrast, melanocytes in adult murine skin are generally confined to hair follicles, with the exception of rare epidermal melanocytes found in the ears, footpads, and tail (1). A few dermal melanocytes may also be found in mice, mostly in the ears. These differences have compromised the use of the mice as a model system for investigation of human cutaneous biology.
It has been discovered that melanocyte migration and development, as well as the survival of melanocytes and mast cells, are dependent on expression of the kit protein, a receptor tyrosine kinase encoded by the c-kit proto-oncogene (2-6). The ligand for kit, known as stem cell factor (SCF) (also called mast cell growth factor, steel factor, and kit ligand) may be produced locally in human skin by epidermal keratinocytes, fibroblasts, and endothelial cells (7-8). However, definitive studies of SCF production in murine skin have not been reported. Transgenic studies using the SCF gene promoter region and beta-galactosidase as a reporter gene suggest that, unlike in human skin, postnatal murine cutaneous SCF expression is limited to the dermis and hair follicles, and not found in epidermal keratinocytes (9). The difference in SCF expression between human and murine epidermis could explain the difference in melanocyte distribution and other biological phenomena in these two species.
SCF may be produced in two isoforms by alternate splicing of exon 6. One isoform lacks exon 6 encoded sequences and exists predominantly as a membrane-bound molecule. The other isoform contains exon 6 encoded sequences which include a protease sensitive site (10-19). Cleavage at the protease sensitive site causes the release of a soluble, bioactive form of SCF. The membrane-bound and soluble forms of SCF have differential effects on melanocyte precursor dispersal and survival (20) and exogenous soluble SCF may produce cutaneous mast cell hyperplasia and cutaneous hyperpigmentation (21-23). In addition, local high concentrations of soluble SCF have been found in lesions of human cutaneous mastocytosis, a disease characterized by dermal accumulations of mast cells and increased epidermal melanin (7, 8, 24) and in spongiotic dermatitis, a common inflammatory condition of human skin (our unpublished data).
The present invention provides a method of identifying a composition, a compound, or a procedure which can produce a skin response in a subject, comprising: a) administering said composition or compound, or applying said procedure to the transgenic mice which express endogenous epidermal stem cell factor, and b) analyzing the skin of said transgenic mice for response. The present invention also provides a method of identifying a composition, a compound, or a procedure which can reduce or treat skin response in a subject, comprising: a) administering said composition or compound, or applying said procedure to the transgenic mice which express endogenous epidermal stem cell factor and which had been induced to produce a skin response and b) analyzing the skin of said transgenic mice to determine the reduction of skin response, wherein the reduction of skin response indicates that the composition, compound or procedure can reduce skin response.
The present invention further provides a method of identifying a composition, a compound, or a procedure which can reduce radiation damage to the skin of a subject, comprising: a) administering said composition or compound, or applying said procedure to the transgenic mice which express endogenous epidermal stem cell factor, b) subjecting the skin of said transgenic mice and the skin of the control transgenic mice to radiation, and c) analyzing the effects of said composition, compound, or procedure on reducing skin radiation damages. The present invention also provides a pharmaceutical composition for treating human skin diseases, comprising (a) a compound that can treat skin diseases of the transgenic mice which express endogenous epidermal stem cell factor, and (b) a suitable carrier, wherein the compound specifically targets the epidermal stem cell factor or its receptor.